Project Summary Alcoholic liver disease (ALD) causes 48% of cirrhotic deaths derived from hepatitis. Increased reactive oxygen species (ROS) production is a major pathogenic factor leading the progression of ALD from steatosis to alcoholic hepatitis (AH). However, clinical trials testing compounds that unselectively scavenge ROS or enhance acquainted antioxidants were unsuccessful. This lack of success supports that ALD disrupts endogenous antioxidant systems inside mitochondria, an organelle formed by two membranes that limit the entry and action of the compounds tested. Our proposal stems from our identification of a mitochondrial antioxidant and redox system impaired in ALD and previously unknown in liver. This system is constituted by the mitochondrial inner membrane ATP binding cassette transporter ABCB10, which exports a previously unknown cargo through its ATP hydrolysis activity (ATPase). Our preliminary data supports that ABCB10 decreases ALD severity by decreasing ROS-mediated damage, as liver-specific deletion of ABCB10 exacerbates hepatic oxidative damage and ALD severity in mice. We hypothesize that ABCB10 transport activity protects from ALD by limiting EtOH-induced oxidative damage. Remarkably, our new data shows that ABCB10 content is decreased in late- stage ALD, including mice and humans with AH. Mechanistically, we show for the first time that ABCB10 exports Biliverdin (BV) from the mitochondrial matrix to the cytosol, where biliverdin reductase (BLVR) is located. Exported BV is used by cytosolic BLVR to regenerate intracellular Bilirubin (BR) destined for ROS scavenging. We hypothesize that ABCB10-mediated BV export is decreased in ALD, shrinking the intrahepatocyte BR pool that protects from ROS-mediated damage. Accordingly, our data show that: i) EtOH and ABCB10 deletion reduce intrahepatocyte BR levels and ii) ABCB10 deletion causes mitochondrial BV accumulation and increases intracellular ROS levels. Thus, we will: 1) Determine the role of ABCB10 in ALD in vivo and 2) Determine the mechanism by which ABCB10 modulates oxidative stress in ALD.